Aramid-epoxy composite internal fixation plates: a pilot study.

Mechanical tests were conducted on an aramid-epoxy composite laminate in vitro and in vivo to determine its suitability for internal fixation plates. This material, fashioned into blank test coupons the size of the standard 4-hole AO-ASIF plates, had a tensile modulus of elasticity significantly lower than bone. In three-point bending, blank test coupons exhibited a low yield strength that would limit utility in significant load-bearing situations, but changes in the layer configuration of the composite could be expected to improve this characteristic. Under destructive loads, these specimens appeared to be less subject to catastrophic failure than carbon fibre composites. Using 4-hole test coupons fastened to a plastic tube simulating bone, four-point bending tests showed that strain-shielding was significantly reduced by aramid composite relative to carbon fibre composite or metal plates. Finally, in-vivo tests on canine femora demonstrated that aramid composite plates were well tolerated and caused less strain shielding during weightbearing, but significant differences in cortical atrophy and porosity beneath steel versus aramid plates were not apparent. Although the plates were relatively flexible, they could not be preformed during surgery like a metal plate.

A comparative analysis of streaming potentials in vivo and in vitro.

Streaming potentials (SPs) measured in vivo at a specific site on intact cortical bone (canine tibia) have been compared with measurements from the same site in vitro, tested as an excised bone strip soaked in Hank's balanced salt solution. The amplitude of SPs per periosteal strain in vitro was larger in 13 tibias than in vivo (by an average x6.5 at 1 Hz), but values per transcortical strain difference were similar. In vitro, SP magnitudes rose more sharply to an asymptotic value as a function of bending frequency than did in vivo signals, possibly because of a difference in the internal state of canaliculi and/or Haversian systems. Similarly, SP response to step-loading decreased to zero more slowly with time in vitro than in vivo. Difficulties encountered in preliminary measurements due to electrical shunting through electrolyte and soft tissues suggest the need for caution in using both in vivo and in vitro SP measurements to extrapolate to electric field strengths on the cellular level.

Transcortical streaming potentials are generated by circulatory pressure gradients in living canine tibia.

Electrical potentials associated with the pulse pressure have been observed in a canine tibia model in vivo. As the medullary pressure rises during pulsing, the periosteal bone surface becomes positive with respect to the endosteal surface. This pattern is consistent with streaming potentials generated by outward flow of fluid through bone with a negatively charged matrix (negative zeta potential). Both the medullary pressure and electric potential oscillations are halted by occlusion of the femoral artery. Furthermore, systemic administration of epinephrine decreases the amplitude of the medullary pressure and the electric potential by the same fraction. Streaming potentials generated by blood flow are distinct from those generated by mechanical deformation and may have additional significance in relation to fracture healing and/or etiology of osteoporosis.

An improved design of electrodes for measurement of streaming potentials on wet bone in vitro and in vivo.

Streaming potentials are generated by mechanical stress in wet bone and may constitute a control mechanism for bone remodeling. Measurement of streaming potentials in bone has attracted considerable effort in past years but quantitative studies have been hampered by relatively poor repeatability when using Ag.AgCl electrodes which contact bone via a wick moistened with electrolyte. Improvement now has been achieved with an electrode design that limits the specific area of contact of an agar/salt bridge by means of a silastic seal, thus permitting the same equipotential surface to be contacted for each set of measurements. This reduces variations caused by bone structure and impedance, and facilitates quantitative comparisons of the response of bone samples to selected variables. The new design also permits considerable qualitative improvement in recordings made from bone during locomotor function in experimental animals in vivo.

Pressure sores: a biochemical test for early detection of tissue damage.

This study was undertaken to seek biochemical indicators in blood associated with the onset of early tissue damage which may progress to the formation of a pressure sore. Changes in serum creatinine phosphokinase (CPK), inorganic phosphate, and lactate dehydrogenase in systemic blood of pigs were investigated before, during, and after local indentation. Pressures of 540mmHg and 700mmHg were applied for six hours on both sides of the scapula and backs of five anesthetized animals. Two hours after release of indentation, serum CPK levels showed marked elevation and remained elevated even after one week. The elevated levels of CPK were shown to correspond to defined pathology of the tissue as determined histologically. On the other hand, inorganic phosphate began to decrease after release of indentation and recovered to the preindentation level after one day. Lactate dehydrogenase did not change significantly throughout the experiment. These results offer important evidence of the potential of CPK as a systemic indicator of muscle damage at an early stage of pressure sore formation.

External ultrasound can generate microampere direct currents in vivo from implanted piezoelectric materials.

Under development is an internal fixation plate that incorporates a piezoelectric element to generate current when excited mechanically by either weight bearing or external application of ultrasound. The intent is to deliver this current to electrodes at a fracture or osteotomy site to aid in prevention or treatment of nonunion. The present study examines quantitatively the ability of external ultrasound to generate current from small piezoelectric ceramic elements implanted in tissue. An ultrasonic transducer (2.25 MHz, 10-20 V input, less than 10 mW/cm2 output) was employed to excite small test coupons of a piezoelectric ceramic in vitro and in vivo with various materials, including water, PVC gel, cortical bone, and living soft tissues, interposed. In all instances, it was possible to generate currents of up to 20 microA after rectification; currents up to 1 mA were achieved in some cases. The work indicates that external ultrasonic energy could effectively power small internal devices designed to stimulate bone healing, without the need for implanted batteries or percutaneous leads.

Design considerations in development of a prototype, piezoelectric internal fixation plate: a preliminary report.

The piezoelectric internal fixation plate represents a new concept in orthopaedic implants. The purpose of this device is to provide stable bone fixation while delivering internally generated, microampere direct currents to prevent or treat nonunion of a fracture or osteotomy. Clinically, currents of this type have been effective in treatment of nonunion, but application has required separate, implanted, or external battery or radiofrequency powered circuits. The "piezoplate" being developed contains an integral piezoelectric element that generates current in response to either physiological loading such as weightbearing or to externally applied ultrasound. Currents are processed by a rectifying circuit for delivery to bone by electrodes. Specially designed series/parallel piezoelectric elements and dual processing circuits are required to generate optimum rectified currents from the low-frequency, high-voltage signals generated by weightbearing, as well as the high-frequency, low-voltage signals produced by ultrasound. This paper reports on the current status of development and describes design parameters of this device which combines the modalities of mechanical fixation and electrical stimulation in a single implant.

Development of a modular wheelchair cushion for spinal cord injured persons.

This paper discusses a modular wheelchair cushion system intended for patients at moderate or low risk for developing pressure sores. With the use of components with different mechanical and physical properties the modular cushion produces improved performance compared with cushions comprising the individual components alone. In addition to achieving clinically acceptable interface pressures, this approach to wheelchair cushion prescription helps to accommodate individual preferences associated with stability, temperature dissipation, and resiliency. Mechanical tests were performed to demonstrate the underlying principles of the modular cushion. Tests with able-bodied subjects identified specific cushion configurations for different weight groups, offering alternative material configurations that may be selected at the discretion of the therapist or patient.

Piezoelectric internal fixation devices: a new approach to electrical augmentation of osteogenesis.

Prototype testing has been accomplished on a piezoelectric, internal fixation plate. This device combines a piezoelectric material with an internal fixation device as an integrated structure that provides mechanical stability, together with self-generated electrical stimulation, for treating fractures and nonunion. In bench and animal tests we have demonstrated that cyclical loading can cause a device of this type to generate electrical charge while attached to bone. After rectification, direct currents within the range known to stimulate osteogenesis can be produced by weight-bearing loads. Furthermore, electrical output of the implants can be increased by externally applied ultrasonic energy. These twin developments add significantly to the potential armamentarium of devices to enhance bone healing.

Ultrasound mapping of the buttock-cushion interface contour.

For patients susceptible to decubitus ulcers, wheelchair cushions and other body support surfaces must be prescribed with a view to providing a uniform distribution of buttock-cushion interface pressures. The capacity to distribute pressure uniformly at the support interface depends on the shape of the bony prominences, and the amount and mechanical compliance of the overlying soft tissue: an indication of this capability may be given by the indentation contour of the buttock-cushion interface. Using normal subjects, we devised a method for measuring this contour using ultrasonic dimension gauging techniques. Because the technique can describe the geometry of soft tissues deformed under load, it could make an important contribution to successful employment of analytical modeling to predict internal tissue stresses during sitting.

A comparison of sitting pressures on wheelchair cushions as measured by air cell transducers and miniature electronic transducers.

With increasing attention to cushion selection for patients subject to decubitus ulcers, greater efforts are being made to prescribe cushions based on individual need. Pressure measurements during sitting frequently are utilized as a clinical guide to the process of selection and fitting. To determine the relationship between pressure values recorded from different types of transducers, simultaneous measurements were made beneath the ischial tuberosities using a Scimedics air cell transducer on one side and a matrix of 5 Kulite electronic transducers on the other side of a subject while sitting on a series of 21 commercially available wheelchair cushions. Although these transducers are different in structure and function, statistically similar results were obtained. The air cell type of transducer appear to be more appropriate for routine clinical use, but caution is advised regarding interpretation of results in terms of absolute pressures due to the many variables involved -- these techniques are suitable primarily for comparative measurements to obtain the optimal seating support for a given patient.